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United States Patent |
5,678,324
|
Viitaniemi
,   et al.
|
October 21, 1997
|
Method for improving biodegradation resistance and dimensional stability
of cellulosic products
Abstract
A method for improving the resistance of cellulosic products against mold
and decay, as well as for enhancing the dimensionally stability of the
products is disclosed. The cellulosic products are subjected to heat
treatment, which is carried out at an elevated temperature. The products
are obtained by drying to a moisture content of less than 15%, and keeping
the resulting products in a moist atmosphere at a temperature of at least
about 150.degree. C. for 2 to 10 hours until a weight loss of at least 3%
has been obtained.
Inventors:
|
Viitaniemi; Pertti (Helsinki, FI);
Jamsa ; Saila (Espoo, FI);
Ek; Pentti (Espoo, FI);
Viitanen; Hannu (Espoo, FI)
|
Assignee:
|
Valtion Teknillinen Tutkimuskeskus (Espoo, FI)
|
Appl. No.:
|
545791 |
Filed:
|
November 9, 1995 |
PCT Filed:
|
May 13, 1994
|
PCT NO:
|
PCT/FI94/00190
|
371 Date:
|
November 9, 1995
|
102(e) Date:
|
November 9, 1995
|
PCT PUB.NO.:
|
WO94/27102 |
PCT PUB. Date:
|
November 24, 1994 |
Foreign Application Priority Data
| May 12, 1993[FI] | 932162 |
| May 11, 1994[FI] | 942209 |
| May 11, 1994[FI] | 942210 |
Current U.S. Class: |
34/396; 34/497 |
Intern'l Class: |
F26B 007/00 |
Field of Search: |
34/396,411,412,446,474,475,494,497
|
References Cited
U.S. Patent Documents
1678559 | Jul., 1928 | Devine et al. | 34/411.
|
2904893 | Sep., 1959 | Willey | 34/446.
|
3811200 | May., 1974 | Hager | 34/337.
|
4176464 | Dec., 1979 | Randolph | 34/412.
|
4182048 | Jan., 1980 | Wolfe et al. | 34/396.
|
4663860 | May., 1987 | Beall | 34/396.
|
5451361 | Sep., 1995 | Ruyter et al. | 264/322.
|
Foreign Patent Documents |
784378 | Jul., 1936 | FR.
| |
2263758 | Jan., 1975 | DE.
| |
2916677 | Nov., 1980 | DE.
| |
WO9006840 | Jun., 1990 | WO.
| |
Primary Examiner: Sollecito; John M.
Assistant Examiner: Gravini; Steve
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak, and Seas
Claims
We claim:
1. A method for increasing the resistance of a cellulosic product against
mold and decay and for improving the dimensional stability of said product
comprising the steps of:
(A) subjecting a wet cellulosic product to a heat treatment so as to reduce
the moisture content of the product to less than 15%; and
(B) subjecting the resulting cellulosic product of step (A) to an
atmosphere saturated with steam at a temperature above 150.degree. C. so
as to reduce the weight of the product at least 3% by decomposition of
wood components present in the cellulosic product.
2. The method according to claim 1, wherein during step (A), the difference
between the inner temperature of said product and the outer temperature of
said product is maintained at about 10.degree. to 30.degree. C. so as to
prevent cracking.
3. The method according to claim 2, wherein step (A) is carried out in the
presence of steam.
4. The method according to claims 2 or 3, wherein step (A) comprises the
steps of:
(i) placing said product in a drying oven, wherein the temperature of said
oven is raised to at least 90.degree. C., and wherein said oven is kept at
said temperature until said product has at least approximately reached
said temperature,
(ii) gradually increasing the temperature of said oven while maintaining
the difference between the inner temperature of said product and the
temperature of said oven at less than 30.degree. C. until the desired
moisture content of the product has been reached, and optionally,
(iii) gradually lowering the temperature of said oven while maintaining the
difference between the inner temperature of said product and the
temperature of said oven at less than 30.degree. C. until the inner
temperature of said product has reached the desired temperature.
5. The method according to claim 4, wherein the temperature of said oven is
raised to at least 100.degree. C.
6. The method according to claim 1, wherein step (A) is carried out at
180.degree. to 250.degree. C. for 1 to 20 hours.
7. The method according to claim 6, wherein step (A) is carried out for
about 2 to 10 hours.
8. The method according to claim 1, wherein step (A) is carried out
essentially under non-pressurized conditions.
9. The method according to claim 1, wherein said cellulosic product is
selected from the group consisting of wood pillars, wood logs, sawn wood,
wood veneer, plywood, wood chips, saw dust and wood fibers.
10. The method according to claim 9, wherein said cellulosic product is
pine wood, and wherein in step (B), said pine wood is subjected to a
temperature of 200.degree. to 250.degree. C. for 2 to 8 hours so as
protect against decay.
11. The method according to claim 9, wherein said cellulosic product is
spruce wood, and wherein in step (B), said spruce wood is subjected to a
temperature of 175.degree. to 210.degree. C., for 2 to 8 hours so as to
protect against decay.
12. The method according to claim 9, wherein the cellulosic product is
birch wood, and wherein in step (B), said birch wood is subjected to a
temperature of 200.degree. to 250.degree. C., for 2 to 8 hours so as to
protect against decay.
13. The method according to claim 9, wherein the cellulosic product is
larch-tree wood, and wherein in step (B), said larch-tree wood is
subjected to a temperature of 200.degree. to 250.degree. C., for 2 to 8
hours so as to protect against decay.
14. The method according to claim 1, wherein in step (B), the weight of the
product is reduced at least about 5%.
Description
The present invention concerns a method according to the preamble of claim
1 for improving the resistance of cellulosic products against mould and
decay as well as to enhance the dimensional stability of the products.
According to a method of the present kind the cellulosic product is
subjected to a heat treatment carried out at an elevated temperature.
It is well-known in the art that the dimensional stability of wood can be
improved by heat treatments. As far as the prior art is concerned,
reference is made to, for instance, the Finnish Patent Specification No.
68,122, which discloses a method for treating wood products at
temperatures of 160.degree. to 240.degree. C. and at pressures of 3 to 15
bar. As a result of the treatment, the capability of wood to absorb water
and thus to expand is considerably reduced. The effect of heat treatments
on decay resistance of wood has also been studied. Mailun, N. P. and
Arenas, C. V describe in their article "Effect of heat on natural decay
resistance of Philippinean woods" (Philippinen Lumberman, Vol 20, No. 10,
1974, p. 18-19, 22-24) the treatment of Asian wood species in dry state at
temperatures of 90.degree., 110.degree., 130.degree., 150.degree. and
175.degree. C. for 240 hours. As a result of the treatment the colour of
the wood changes to chocolate brown. An extended treatment at 130.degree.,
150.degree. and 175.degree. C. increased the resistance of the wood
samples against two brown rot fungi. However, at the same time it made the
wood weaker.
Because all kinds of wood are not suited for the conventional pressure
impregnation methods, using substances which prevent the growth and
spreading of fungi, heat treatment is an interesting alternative for
protecting wood against decay.
The prior heat treatment processes, which call for the use of pressure and
extended treatment times, have been too complicated for industrial
applicability. It has also been ascertained that under the influence of
high pressures and increased temperatures wood becomes brittle and it
weakens. Furthermore, wood is easily ignited at high temperatures.
The present invention aims at eliminating the problems related to the prior
art and to provide a completely novel solution for improving the
dimensional stability of and the resistance against decay and mould (i.e.
the biodegradation resistance) of cellulosic products.
The invention is based on the concept of carrying out the heat treatment of
the cellulosic product in two stages: first the product is dried to
desired moisture content, typically to below 15%. Then the temperature is
rapidly raised above 150.degree. C. (typically to about 180.degree. to
250.degree. C.) and the treatment is continued at that temperature until
the weight loss of the treated product amounts to at least 3%.
In particular the method according to the invention is principally
characterized by what is stated in the characterizing part of claim 1.
As mentioned above, unseasoned timber or similar cellulosic products are
used as starting materials for the method according to the present
invention. The product can be dried at any suitable conditions (even
outdoors at ambient temperature) to the desired moisture content of less
than 15%. According to a preferred embodiment of the invention the product
is, however, dried at elevated temperatures. The colour of the wood
product will become darker during such drying. In connection with the
drying due care is taken to avoid cracking of the product. This goal is
advantageously attained by constantly determining the temperatures of the
interior and the surface, respectively, of the wood and by maintaining the
temperature difference at a reasonably small value. Preferably said
difference amounts to about 10.degree. to 30.degree. C. This procedure is
followed both when the temperature is raised and when it is lowered.
Surprisingly, it has been found that said solution will even completely
prevent the formation of cracks in (the interior parts of) the wood
material. When larger amounts of wood are to be dried several samples
should be provided with sensors. On an industrial scale, the preferred
procedure comprises determining for each kind of timber a heating
programme of its own which takes into account the influence of the initial
moisture content on the process.
In order to protect the wood and improve heat transfer during drying it is
preferred to use steam during drying. According to a preferred embodiment,
the drying process according to the invention comprises the following
steps:
a) first the temperature of the drying oven is raised to at least about
90.degree. C., preferably to at least 100.degree. C., and that temperature
is maintained until the wood has at least approximately reached the same
temperature,
b) then the temperature of the oven is gradually raised so that the
difference between the interior temperature of the wood and the
temperature of the oven does not exceed 30.degree. C. until the desired
moisture content of the wood is reached, and
c) finally, the temperature of the oven is lowered gradually while ensuring
that the difference between the temperature of the interior parts of the
wood and the temperature of the oven does not exceed 30.degree. C. until
the interior of the wood has reached the desired temperature.
If needed, stage c can be left out. The heat treatment, which will be
described in more detail below, is then carried out immediately after
stage b.
During the first stage of the present invention (step a), the temperature
of the drying oven is preferable set at a value of about 100.degree. to
150.degree. C., preferably 100.degree. to 120.degree. C. In the second
stage (step b), the heating is stopped when the humidity of the wood is
below 15%, e.g. 1 to 15%. During step b and step c, if any, the difference
between the external temperature and the interior temperature of the
cellulosic product is kept at a value of 10.degree. to 30.degree. C. Too
small a temperature difference prolongs the drying process, whereas too
large a difference increases the risk of internal cracking. During stage c
the temperature of the oven is lowered until the interior temperature of
the wood has decreased below 100.degree. C.
During stages a, b, and c water steam is fed into the oven to keep the wet
temperature at about 80.degree. to 120.degree., preferably at about
100.degree. C. It is preferred to use saturated water steam.
When the moisture content of the product has dropped to below 15%, as a
result of the drying, the treatment is continued at an elevated
temperature.
During the second stage of the process the temperature is kept higher than
during the first stage of the process. It is preferred to operate the
process at about 180.degree. to 250.degree. C. in an atmosphere of
saturated steam. The temperature can also be raised during the second
stage, as will appear from Example 2. The duration and the temperature of
the treatment are interdependent, as explained in connection with Example
1. Typically, the heat treatment of the second stage takes at least some
0.5 hours, preferably 1 to 20 hours and in particular about 2 to 10 hours.
The weight loss of the product can be adjusted by varying the heat
treatment. This makes it possible to change the strength and decay
resistance properties of the product as desired. Therefore, the heat
treatment is continued until a weight loss of at least 3% (based on dry
matter) has been obtained. Clear improvements of the dimensional stability
of the product are reached at this value already. Mould and decay
resistance will also be improved, and further improvements of said
properties can be obtained by continuing the heating until at least a
about 5%, preferably at least a 6 or even a 8%, weight loss has taken
place in the product.
Summarizing, the features obtained by the present invention are:
Improvement of decay resistance (in comparison to wood which natively has a
good resistance to decay)
Improvement of mould resistance
Improvement of the dimensional stability
Removal of pitch
Heat conductivity decreased by 25-40%
Improvement of paint adherence
The heat treatment of the second stage is, according to a preferred
embodiment of the invention, carried out at least essentially under
non-pressurized conditions, i.e. at atmospheric pressure.
The method according to the invention is suited for treatment massive wood
goods, such as logs and pillars. In addition, the method can be applied to
veneer, chips, saw dust, wood fibres and other cellulosic products, such
as, for instance, crates.
The wood preservation effect that can be produced is studied in more detail
in Example 2. However, in this connection it should be pointed out that
good protection against decay requires that dried sawn timber of pine is
kept for preferably about 2 to 8 hours at a temperature of 200.degree. to
250.degree. C. The same conditions are used for birch and larch-tree,
whereas good protection against decay can be obtained at slightly lower
temperatures for spruce. Thus, spruce can be treated, for instance, at
about 175.degree. to 210.degree. C. The method is well-suited for
treatment of aspen.
Example 3 explains in detail the decrease of heat conductivity as a result
of a treatment carried out according to the invention.
The invention provides considerable advantages. It will therefore provide
for a shortening of the time required for drying of wood. The colour
changes appearing during drying can be utilized and, at the same time, the
resistance of wood against decay and mould and the dimensional stability
can be improved. Detrimental pitch can be removed from samples of
coniferous wood by the treatment. As examples of products that can be
treated with the method according to the present invention, the following
can be mentioned: external cladding, window frames, outdoor furniture, and
boards for sauna platforms.
After a treatment according to the present invention, the dimensional
instability under the influence of moisture is reduced by 50 to 70%. The
resistance against decay of the products is improved. At its best, the
resistance is on the same level as that obtained by pressure impregnation
or even better without any substantial weakening of the strength
properties of the products. The treated wood forms a good surface for
paint.
The preparation process is simple and quick (short treatment times) and
there is no need to use pressure. As far as its weathering resistance,
resistance to decay and mould, and strength properties are concerned, the
product can be modified in a controlled manner by the method. The method
is suitable for all kinds of wood. By means of the heat treatment it is
become possible also to improve the properties of the heartwood, which
cannot be done by pressure impregnation. The durability of those kinds of
wood which are difficult to impregnate can be improved. The improvement of
the permeability of wood makes it possible to impregnate the wood with
other colouring agents.
In the following the invention will be examined in greater detail with the
help of the attached drawings and some working examples.
FIG. 1 is a simplified schematic representation of the construction of an
apparatus which can be used for carrying out the present invention,
FIG. 2 indicates the influence of the temperature and treatment time on the
weight loss of the product,
FIG. 3 indicates the reduction of tangential swelling of the wood sample as
a function of the weight loss,
FIG. 4 indicates the reduction of radial swelling of the wood sample as a
function of the weight loss,
FIG. 5 indicates the reduction of moisture taken up by the wood sample as a
function of the weight loss,
FIG. 6 indicates the changes of bending strength caused by the heat
treatment,
FIG. 7 shows the moisture contents of bending test samples after
conditioning for 4 weeks,
FIG. 8 shows the weight losses of heat treated and control samples,
respectively, after decay testing,
FIG. 9 shows the drying of unseasoned spruce according to a preferred
embodiment according to the present invention,
FIG. 10 indicates the weight losses of veneer as a function of the duration
of the heat treatment,
FIG. 11 indicates the reduction of thickness swelling of plywood as a
result of a heat treatment, and
FIG. 12 indicates the reduction of the moisture content of plywood under
the influence of a heat treatment.
An apparatus shown, for instance, in FIG. 1 is used in the present
invention. The apparatus comprises an oven 2 surrounded by an oven jacket
1. The samples 3 are placed in the oven, which is provided with inlet 4
and outlet 5 channels for air 5, for conducting moist air through the
oven. The outlet channel 5 is combined with a steam feed pipe 6 for
feeding more water steam into the outlet air coming from the oven. In
order to form a closed cycle the inlet and outlet channels are joined each
to its end of a set of ducts 7 provided with a fan 9 and with heating
means 8. The air flowing through said ducts are heated by electric
resistances 8 to the set temperature and conducted via the fan 9 to the
inlet channel 4 of the oven. The recycling direction of the air in the
apparatus is indicated with an arrow.
By using the present apparatus it is possible to make sure that the samples
placed in the oven are heated to the desired temperature by moist air. By
changing the amount of steam, which is being fed, the moisture content of
the air can be altered. Usually, the air of the oven is saturated with
water steam.
EXAMPLE 1
Heat treatment of wood
Moist wood is dried in the above-described apparatus at 120.degree. to
140.degree. C. either with steam or without it. As a result of the
treatment, there is some darkening of the colour of the wood sample, but
no cracking. When the moisture content of the wood is below 15% the
temperature is raised to at least 175.degree. C., preferably to
180.degree. to 250.degree. C. The treatment is continued for 2 to 10
hours. Saturated steam is conducted to the apparatus. By varying the
temperature and the time, the desired result can be obtained. The colour
of the wood darkens further.
FIG. 2 shows the influence of temperature and time on the reduction of wood
weight.
By adjusting the weight losses the properties of the wood can be changed as
desired. FIGS. 3, 4 and 5 depict the reduction of tangential swelling of
the wood, the reduction of the radial swelling of the wood, and the
reduction of the amount water absorbed by the wood (wood moisture content)
in comparison to the control samples. The graphs of FIGS. 4 and 5
correspond to graph model of FIG. 1.
The heat treatment weakens the bending strength of wood after a certain
weight loss. On the other hand, the experiments show that the bending
strength properties of some of our samples were even better than the
corresponding properties of the control samples (FIG. 6). This is due to
the fact that, depending on ambient humidity, some of the heat treated
samples clearly adsorbed less water than the control sample (FIG. 7).
EXAMPLE 2
Decay test
The decay test was carried out according to European Standard EN 113
modified as follows: the number of parallel test specimens was four, the
sizes of the test specimens were 5.times.20.times.35 mm, and they were not
rinsed before the test. The samples were subjected to the test rot fungus,
cellar fungus (Coniophora puteana), for 2, 4, 8, and 12 weeks.
The test specimens were sawn from planks of pine, birch, larch-tree and
spruce, treated according to example 1. Table 1 contains a summary of the
conditions prevailing during the heat treatment.
TABLE 1
______________________________________
Treatment conditions of the test specimens of the decay test
Sample Wood species Heat treatment (time/temp.)
______________________________________
1 Pine 1.5 h/200.degree. C. + 1 h/220.degree. C.
2 Pine 2 h/220.degree. C.
3 Pine 4 h/210.degree. C.
4 Birch 1 h/160.degree. C.
5 Birch 4 h/210.degree. C.
6 Birch 1 h/160.degree. C. + 2 h/220.degree. C.
7 Birch 1 h/160.degree. C. + 4 h/220.degree. C.
8 Larch-tree 4 h/210.degree. C.
9 Spruce 4 h/210.degree. C.
10 Spruce 1 h/180.degree. C.
______________________________________
After the heat treatment the dry matter of the wood specimens were
determined. The test specimens were sterilized by radiation (Co-60), the
sterilized test specimens were inserted in kolle dishes on a fungus
culture growing on malt agar medium. At least one heat treated test
specimen and one untreated control sample were inserted into each dish.
At the end of the decay test the specimens were dried at 103.degree. C. and
the weight losses of the specimens were calculated according to EN 113.
For pine a weight loss of less than 10% was achieved by the heat
treatment; the weight losses for untreated wood were over 30%. The
smallest weight losses for heat treated birch, larch-tree and spruce were
close to zero.
The results of the decay tests are indicated in FIG. 8. It is apparent from
the figure that a mild heat treatment (160.degree. C.) does not yet
significantly improve the decay resistance of the timber.
EXAMPLE 3
Drying of unseasoned spruce
When a specimen of wet spruce (50.times.100.times.1500 mm), initial
moisture content about 40%, was heated according to the preferred drying
embodiment of the invention for 24 hours by operating the drying system in
such a way that the difference between the internal and external
temperatures was 10 to 20 degrees, no cracks were found in the test
specimen (FIG. 9). The final moisture of the dry test specimen was below
5%.
EXAMPLE 4
Reduction of heat conductivity
Table 2 presents the heat conductivities of heat treated samples of spruce,
pine and aspen. The table also indicates the conditions of the heat
treatment.
TABLE 2
______________________________________
Heat conductivity of the test specimens
Density at time
Wood/treatment,
of measuring,
Heat conductivity
temperature and time
kg/m.sup.3 .lambda..sub.10, W/(mK)
______________________________________
Aspen, control 415 0.098
Aspen, 4 h 210.degree. C.
403 0.077
Aspen, 10 h 210.degree. C.
379 0.077
Spruce, control
497 0.11
Spruce, fresh, 26 h,
375 0.086
heat treatment: 3 h/
220.degree. C.
Spruce, 8 h 230.degree. C.
399 0.080
Pine, control 583 0.13
Pine, fresh, heat
520 0.107
treatment: 3 h 220.degree. C.
Pine, 30 h 230.degree. C.
476 0.088
______________________________________
EXAMPLE 5
Birch veneer, thickness 1.5 mm, was heat treated in an oven of the kind
shown in FIG. 1. The temperature of the treatment was 200.degree. C. and
the time 2 to 7 hours.
The test specimens were selected by dividing the veneer into two parts and
by choosing one half of the veneer as a control. The other half was heat
treated. A 3-ply plywood was prepared from the veneer. The gluing was made
by FF glue, which was applied to the surfaces of the veneer by a brush.
The veneers were pressed together at 130.degree. C. for 6 minutes. The
compression load was 1.7 MPa. The control plywood and the plywood prepared
from the heat treated veneers were kept in the same pressing.
In order to determine the thickness swelling, the test specimens were dried
in an oven at 102.degree. C. Then they were immersed into 20.degree. C.
water for 2, 6, 26, and 168 hours. The test specimens were prepared for
the strength testing by conditioning them at a relative humidity of 65%,
whereinafter they were evaluated for wood failure, tensile strength and
bending strength. The tests included two parallel test specimens.
The weight loss of the wood (calculated on basis of the dry matter) caused
by the heat treatment is indicated in FIG. 10. As a result of the
treatment the weight of the wood decreased by 3.4 to 8.4%.
The thickness swelling of the plywood is indicated in FIG. 3.
TABLE 3
______________________________________
Thickness swelling of plywood and moisture content
of plywood after immersion into water
Thickness swelling Moisture content
Sample 2 h 6 h 27 h 168 h
2 h 6 h 27 h 168 h
______________________________________
2 h heat
2.9 5.5 10.2 11.7 18.0 29.6 50.0 79.1
2 h control
9.1 11.4 12.3 12.5 38.0 49.5 60.8 72.3
3 h heat
2.7 5.2 8.9 11.0 14.8 27.0 43.2 74.8
3 h control
5.5 9.3 10.9 11.1 28.8 43.5 61.4 75.9
4 h heat
1.8 3.7 7.4 9.9 13.4 23.6 42.0 74.2
4 h control
8.6 11.9 14.5 14.7 28.8 39.3 53.9 66.5
5 h heat
1.9 4.1 8.0 11.2 16.1 27.4 46.6 79.9
5 h control
6.8 12.2 10.9 11.1 34.1 47.3 65.6 78.7
6 h heat
1.8 3.6 6.3 8.6 14.5 25.1 43.0 77.2
6 h control
5.4 8.6 10.4 10.6 29.1 43.3 62.8 79.0
7 h heat
1.2 2.8 5.5 7.9 13.2 22.2 38.0 67.5
7 h control
6.7 9.7 11.1 11.2 31.2 48.8 59.1 73.7
______________________________________
The thickness swelling of the control samples varied to a large extent. For
this reason, the swelling reduction results presented in FIG. 11 have been
calculated in relation to the control samples of each test series. FIG. 12
shows the reduction of the amounts of water absorbed by the wood samples
compared to the untreated samples.
As far as thickness swelling is concerned the best results were obtained by
the treatment having the longest duration, i.e., by a 7 hour heat
treatment. After a 2 hour immersion the thickness swelling was then 80%
smaller than that of the control samples. An almost equally good a result
was reached by a 4 hour treatment. 2 and 3 hour heat treatments reduced
thickness swelling after a 2 hour immersion to 50 or 70%. After a 24 hour
immersion the thickness swelling of plywood which had been heat treated
for 7 and 4 hours was 50% smaller than that of the control samples.
The heat treatment reduces the amount of water absorbed by the wood sample
(=moisture content of wood). Subject to immersion into water for 24 hours,
the moisture content of plywood which had been heat treated for 7 hours
was about 38 smaller than that of the control plywood.
Table 4 indicates the strength properties of the plywood articles.
TABLE 4
______________________________________
Strength properties of the plywood
Shear strength of Moisture
glue line content (%)
Wood Resist. Tensile
during Bending
failure to shear
strength
strength strength/
Sample % N/mm.sup.2
N/mm.sup.2
testing mm.sup.2
______________________________________
2 h heat
21 1.7 65.3 5.0 132
2 h control
97 3.0 81.7 5.3 148
3 h heat
81 2.4 83.1 4.7 155
3 h control
98 3.0 101.7 4.1 148
4 h heat
99 2.2 55.4 5.2 130
4 h control
95 3.0 110.9 5.0 165
5 h heat
100 1.7 50.4 4.2 95
5 h control
84 3.1 74.5 4.7 128
6 h heat
99 1.9 37.5 4.5 108
6 h control
77 2.4 92.8 4.9 139
7 h heat
97 1.8 55.2 4.9 101
7 h control
94 2.9 84.5 5.4 141
______________________________________
Requirements for a 3-ply plywood:
Shear strength of glue line, dry, strength=2.1N/mm.sup.2. If the strength
is less than that, the wood failure percentage should be more than or
equal to 50%.
Tensile strength 54N/mm.sup.2
Bending strength 72N/mm.sup.2
The tensile strength of plywood prepared from heat treated veneer was
almost always less than the required 2.1, but because the wood failure %
exceeded 50, it should be noted that the requirements regarding shearing
strength were nevertheless fulfilled.
The bending strength of the plywood prepared from heat treated veneer was
inferior to that of the control plywood, but even so it met the
requirements. The required bending strength was not reached with heat
treated veneer which had been heat treated for 5 or 6 hours.
EXAMPLE 6
Field trials
Test specimens (50.times.25.times.500 mm) were heat treated for 4 hours at
220.degree. C. The samples were placed on test field in contact with the
earth. After a time of one year the test specimens were checked and
evaluated.
The results were evaluated using the following scale: 1=some beginning
decay (25%), 2=50%, 3=75%, 4=the test specimen breaks under a weight.
Average values of the results:
Pine, control=0.3. Heat treated pine=0.
Spruce control=1. Heat treated spruce=0.2.
Birch, control=3.6. Heat treated birch=2.5.
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